Measurement devices and methods using acoustic signals are well known in the prior art. Acoustic measurement may for example be used to measure distance, depth, volume, flow rate, or acoustic properties of measurement objects such as attenuation and the like. Often the travel time for an acoustic pulse through a medium, or the travel time for an acoustic pulse back and forth from a reflecting measurement object, is used as a basis for calculation of for example the distance to the measurement object. The distance is basically calculated from the very well known formula distance=velocity*time.
However, the velocity of sound is dependant on for example temperature, which may render the measurement erroneous. To overcome this problem, many acoustic measurement systems comprise a reference system, wherein the acoustic pulse travels a known distance in order to determine the current speed of sound, whereby the current speed of sound then is used to calculate the unknown distance or volume etcetera of the measurement object. A reference system is disclosed in for example UK patent application GB2 164 151.
However, the velocity of sound is also dependent on the composition of the gas which the signal travels through. Often the gas composition varies throughout the measurement device, whereby the speed of sound is different at different parts of the measurement device, which may significantly affect the accuracy of the measurement.